The mid-infrared region of the electromagnetic spectrum (wavelengths of 2.5-25 μm; frequencies of 4000 cm−1 to 400 cm−1) is highly interesting in many applications such as spectroscopy, sensing, and thermography. See B. Stuart, Infrared Spectroscopy: Fundamentals and Applications, John Wiley (2004); H. Jane and P. T. Ralph, Measurement Science and Technology 24, 012004 (2013); and M. Carosena and M. C. Giovanni, Measurement Science and Technology 15, R27 (2004). In this frequency range, only a few kinds of light sources are available, mainly quantum cascade lasers and thermal sources (blackbodies). Thermal sources can be inexpensive, but suffer from poor efficiencies which can be as low as 10−4 for spectroscopic applications. See D. Costantini et al., Physical Review Applied 4, 014023 (2015). Because convection losses can be suppressed by operating under vacuum and conduction losses can be suppressed by a proper design, the ultimate efficiency limit for incandescent sources appears to be due to emission into unwanted frequencies and directions. See G. Brucoli et al., Applied Physics Letters 104, 081101 (2014).
The development of novel thermal sources that control the emission spectrum and the angular emission pattern is thus of fundamental importance. In the last few years, research on thermal radiation has led to the achievement of 1) spatially coherent (i.e. directional) or 2) temporally coherent (i.e. narrow-band) thermal sources by using wavelength-scale optical structures. See J.-J. Greffet et al., Nature 416, 61 (2002); N. Dahan et al., Physical Review B 76, 045427 (2007); I. Celanovic et al., Physical Review B 72, 075127 (2005); S. E. Han and D. J. Norris, Opt. Express 18, 4829 (2010); C. Arnold et al., Physical Review B 86, 035316 (2012); J. Drevillon and P. Ben-Abdallah, Journal of Applied Physics 102, 114305 (2007); B. J. Lee et al., Opt. Express 16, 11328 (2008); X. Liu et al., Physical Review Letters 107, 045901 (2011); P. Bouchon et al., Opt. Lett. 37, 1038 (2012); D. L. C. Chan et al., Opt. Express 14, 8785 (2006); J. A. Mason et al., Applied Physics Letters 98, 241105 (2011); and S. Vassant et al., Applied Physics Letters 102, 081125 (2013). A few recent papers even succeeded in combining both properties of directionality and monochromaticity using diffraction order engineering in periodic structures. See D. Costantini et al., Physical Review Applied 4, 014023 (2015); and M. De Zoysa et al., Nat Photon 6, 535 (2012). It is also desirable that thermal radiation sources be capable of rapid modulation.
Structures displaying a so-called epsilon-near-zero (ENZ) mode have been studied as potential thermal sources. See S. Vassant et al., Opt. Express 20, 23971 (2012); and S. Vassant et al., Physical Review Letters 109, 237401 (2012); and S. Campione et al., Physical Review B 91, 121408 (2015). Thermal-radiation control has been demonstrated from such ENZ devices and electrical control of the reflectivity and emissivity have been shown. See Y. C. Jun et al., Applied Physics Letters 105, 131109 (2014); S. Vassant et al., Physical Review Letters 109, 237401 (2012); and S. Vassant et al., Applied Physics Letters 102, 081125 (2013). Very recently, high-speed modulation of thermal emission has been demonstrated using a quantum well stack and a photonic crystal. See T. Inoue et al., Nat Mater 13, 928 (2014).
However, a need remains for a thermal emitter comprising near-zero permittivity materials wherein the frequency (wavelength) and directionality of the emitter can be accurately controlled.